Hello, Hassan.

Could you please explain what you mean by "using authentic materials"? Do you mean "using published research reports" or some other type of "real-life" document?

Many instructors discourage the use of passive voice since it makes sentences less concise; longer sentences are more likely to trip up writers and confuse readers. Clear, concise sentences that go Subject -> Verb -> Object are the basis of good English communication.

Passive voice can be useful when an author is trying to emphasize the action, rather than the actor. If you read research papers, the "Experimental" sections typically contain many sentences in passive voice. E.g. "The solutions were vacuum filtered." or "The ingots were annealed." Note that these sentences don't mention the researchers at all, just the objects of the research and what was done to them.

Problems with passive voice can arise when discussing results, however. A sentence beginning "These data were interpreted to mean..." gives the impression that the interpretation is not that of the author, though we know that this is the case. Some think that passive-voice statements like this are tactful and modest, others think that they are "weaselly" (http://en.wikipedia.org/wiki/Weasel_word).

Jon